Human Genetic Disorders

I. Genes

a. Definition

* Segments of DNA molecules

* A nucleotide sequence

b. Functions

* Control of hereditary traits

* Sequence coding for a polypeptide which maybe an enzyme, or a part of an enzyme, which in turn is responsible for a certain phenotype or trait.

c. The Human Karyotype

* Ordered arrangement of chromosomes arranged in homologous pairs

* The chromosome complement of a cell or organism characterized by the number, size and morphological features of each chromosome

II. Types of Human Genetic disorders

a. Human Chromosomal Abnormalities

1. Nondisjunction

* Occurs when homologous chromosomes fail to segregate during meiosis

* Produces gametes with three chromosome pairs (trisomy) or only one chromosome of a normal pair (monosomy)

* Involves any chromosomes, including the sex chromosomes

* Causes serious genetic disorders

Ex: Down's syndrome- trisomy 21

Edward's syndrome- trisomy 18

Patau syndrome- trisomy 13

2. Klinefelter's syndrome, XXY

* Results when an abnormal sperm, containing both X and Y-chromosomes, fertilizes a normal egg with one X-chromosome

* Can also result when an XX egg is fertilized by a Y sperm

* Offspring has genotype XXY

* Occurs in 1 in 1000 born males

* Have reduced fertility and maybe mentally retarded

3. Turner's syndrome, XO

* Results from the absence of an X-chromosome in an egg

* When fertilized by a normal X-carrying sperm, offspring has the genotype XO ("O" indicating the lack of a chromosome)

* XO occur in 1 in 2000 born females

* They do not mature sexually

* Sterile

* Abnormally short an have webbed necks

4. Triplo-X syndrome, XXX

* Occurs at the frequency of 1 in 1000 live female births

* Do not have distinct clinical features except for tallness and menstrual irregularities, secondary amenorrhea and premature menopause

* Mostly have no sexual abnormalities but are generally of subnormal mental abilities

5. Jacob syndrome or Double Y syndrome, XYY

* Occurs in 1 in 1000 male births

* First observed in 1960's

* Extra Y-chromosome predisposes the male to violent behavior

6. Hermaphrodites

* Have both ovarian and testicular tissues

* Do not have the normal Y; rather they have the minute y

* Mental retardation, congenital anomalies and hypertonia

7. Chromosomal deletions

* Abnormalities in chromosome structure due to some deletions in segments of some autosomal and sex chromosomes

Ex: cri-du-chat syndrome or 5 p sydrome

* Deletion of the short arm of chromosome 5

* Mentally retarded

* With small epiglottis and larynx

* Muscular dystrophy

* Deletion of a small segment of chromosome X

* Involves deletion of genes responsible for synthesis of the protein dystrophin.

b. Human Allelic Disorders (Recessive)

1. Albinism

* not an infectious disease

* cannot be transmitted through contact, blood transfusions, etc

* result of biological inheritance of genetically recessive alleles

* usually occurs with equal frequency in both genders

* have little or no pigment in the eyes, skin, and hair

* approximately one in 17,000 people have albinism

2. Phenylketonuria (PKU)

* an inherited disorder that increases phenylalanine

* varies among ethnic groups and geographic regions worldwide

* inherited in an autosomal recessive pattern

* detected in 1 in 10,000 to 15,000